In the production of flat glass the molten silica-based mix is cooled slowly under carefully controlled conditions. This annealing procedure removes undesirable stresses from the glass. Cooling occurs in an annealing "lehr"; hence, the glass is termed "annealed" or "ordinary" glass. Annealed glass which has been heated to a temperature near its softening point and forced to cool rapidly under carefully controlled conditions is described as "heat-treated glass." The heat treating process produces highly desirable conditions of induced stress (described below) which result in additional strength, resistance to thermal stress, and impact resistance.

Heat-treated glasses are classified as either fully tempered or heat strengthened. According to Federal Specification DD-G-1403B, fully tempered glass must have a surface compression of 10,000 psi or more or an edge compression of 9,700 psi or more. Heat-strength glass must have a surface compression between 3,500 and 10,000 psi, or an edge compression between 5,500 and 9,700 psi. The fracture characteristics of heat- strengthened glass vary widely from very much like annealed glass near the 3,500 psi level to similar to fully tempered glass at the 10,000 psi level.

HEAT TREATMENT PRINCIPLE

Glass can fracture when its surfaces or edges are placed into tension. Under these conditions inherent surface or edge fissures may propagate into visible cracks.

The basic principle employed in the heat treating process of tempered glass is to create an initial condition of surface and edge compression. This condition is achieved by first heating the glass, then cooling the surfaces rapidly. This leaves the center glass thickness relatively hot compared to the surfaces. As the center thickness then cools, it forces the surfaces and edges into compression. Wind pressure, missile impact, thermal stresses or other applied loads must first overcome this compression before there is any possibility of fracture.

Nanjing SunHouse Glass Co., Ltd. (named Nanjing Shunhao Glass Co., Ltd. in the past), specializes in glass processing for building and home use, since its foundation in 1996. Our main products include: Tempered Glass, Tempered safety Glass, Bullet Proof Glass, Low E Glass, Laminated Glass, Insulated Glass, Fire resistant Glass, Low Emissivity Glass…

Each year glass-topped tables account for 20,000 serious injuries. A report in this month's issue of Pediatric Emergency Care concludes that such tables are an overlooked safety threat. Based on research conducted by Children’s Hospital Boston and written in collaboration with Consumers Union, the study found that common glass used in tables can cause severe injuries to children. Researchers concluded that many of these injuries could have been prevented by the use of tempered glass and the development of safety standards requiring its use.

As we explained earlier on this blog, when tempered glass breaks it shatters into many small fragments. But when common glass breaks, it produces large, jagged pieces. “Huge shards of glass are basically like knives. If they sever an artery, they can cause uncontrolled bleeding, and the injury can be fatal,” says Amir Kimia, MD, of the Division of Emergency Medicine at Children’s Hospital Boston, who led the study

“This is a serious safety hazard with a simple remedy,” says Donald Mays, Senior Director of Product Safety and Technical Policy for Consumers Union.  “The use of tempered glass can significantly reduce the thousands of serious injuries incurred each year from the use of common annealed glass in furniture.” 

Tempered glass and standard glass differ in the way they are processed. To manufacture any glass; sand, soda ash and lime are mixed together and melted at very high temperatures. This hot liquid is formed into glass by the methods of blowing, pressing, or drawing. Once glass is formed, it goes through an annealing process where it is reheated and cooled. This keeps it from shattering and restores its strength.

The annealing or cooling process is what determines whether glass will be tempered or standard. Tempered glass is forced to cool very rapidly while standard glass is allowed to cool slowly. Standard glass’s slow cooling produces different properties than those found in rapidly cooled tempered glass. Tempered glass is often called toughened, or strengthened, glass as it can withstand at least four times the pressure of standard glass.

The most notable property of tempered glass is how it shatters or breaks. Standard glass breaks into large, irregularly shaped sharp shards. Tempered glass, shatters into small evenly shaped pieces that pose much less risk of injury to those coming in contact with them.

Tempered glass is more heat-resistant and scratch-resistant than non-treated glass. Outwardly, tempered glass does not appear any different than standard glass. Both types of glass are made in varying sizes and thicknesses and can be colored or tinted.

Standard glass can be cut to size or pressed into shape after it is processed. If an application requires, standard glass’s edges can be polished or holes drilled into it. Strengthened glass can not be reworked after it is tempered. Attempts to cut or drill into a pane of tempered glass would result in it shattering completely.

Because it is stronger and its fracture pattern safer than ordinary glass, tempered glass was once the standard for car windows and glass doors.

Nanjing SunHouse Glass Co., Ltd. is specialized in manufacturing tempered glasses.